mad_jock
9% is nothing to be honest.
The simulated feathered torque is 10% so in an engine failure the plane can have 90% torque split between the engines.
At 9% imbalance they will be well away from V minimum control airborne issues, Vmca.
The roll is just a secondary effect of yaw which was obviously not controlled. And to be honest with the experience level of the FO I can understand the reasons why as well
From the report, #1 went to -9% torque (indicated), then feathered and up to 22% torque / down to 96% RPM.
Torque might normally relate to thrust, but (not being a TP pilot), I suspect not with a feathered prop? Therefore the P2 had to handle whatever the effect was of a quickly / without warning feathered prop on the left - and then 3s later, having corrected, having that feathered engine / asymmetry suddenly restored to full symmetric power.
I am open to education about the relationship of torque to thrust, especially when a prop is feathered in flight with a running engine.
Edit: might not the prop going to feather, but at 100% RPM, produce a significant
increase in thrust on #1 engine? i.e. as prop went from beta (flat) to feather... from
Link
Uncommanded feather
Uncommanded feather is very similar to the overtorque condition noted above. The propeller pitch will abruptly increase, causing a rapid rise in torque with a rapid drop in RPM because the engine is still providing power to the propeller. While the pitch is changing, the thrust may increase and then decrease rapidly. The airplane will have asymmetric thrust